1. Technical Field
The present invention relates to an electrostatic charge image developer, a process cartridge and an image forming apparatus.
2. Related Art
In an electrophotography method, an electrostatic latent image is formed on an image holding member (photoreceptor) by charging and exposing, and then developed by a toner; subsequently, the developed image is transferred onto a transfer receiver, and fixed by heating or the like to obtain an image. Developers used in electrophotographic methods such as this are categorized into single component developers, where a toner obtained by dispersing a coloring agent in a binding resin is used singularly, and two component developers, comprising a toner and a carrier. In two component developers, the carrier performs functions such as agitation, transportation and charging of the developer, and since these functions are separate to those of the developer, the two component developer has excellent controllability and is used widely at present.
In recent years digital processing has been adopted as a means for achieving high image quality, enabling high speed processing of complex images. In this case, as the output image, an electrostatic latent image formed by an optical system must be reproduced with high fidelity, and accordingly, the particle size of toner therein has been steadily decreasing, accelerating activity in the field of high fidelity reproduction. On the other hand, there is a demand for the number of components to be reduced, to achieve miniaturization, and also for the lifetime of consumable articles to be extended, to achieve low costs. That is, there are demands for the developer to have improved functions and reliability. Furthermore, in order to achieve higher productivity, the speed of an image holding member has been increasing; and accordingly, in order to stably obtain high image quality, it has become very important to improve the respective processes of development, transferring, fixing and cleaning.
For example, in a cleaning process in which toner remaining on the photoreceptor is scraped by a cleaning blade, there is a problem in that small and spherical toner particles slip through the cleaning blade and appear on a subsequent image.
Furthermore, as the particle size of the toner decreases, it becomes more important to improve charging characteristics. During developing, as the particle size of the toner decreases, a charge amount per weight (q/m) becomes larger, and developing becomes more difficult. Accordingly, an electric field used for developing needs to be increased in order to obtain the same development amount. However, since the upper limit of this electric field is determined by the surface potential of the photoreceptor, a value greater than a particular potential value cannot be achieved. Therefore, in order to control a charging amount of the toner to obtain a particular development amount, the charging capacity of the carrier has to be controlled to be low, in order to reduce the q/m of the toner. However, when the particle size of the toner decreases, despite an increase in the q/m of the toner, a charging amount per one toner particle decreases. As a result, electrostatic adherence of the toner and carrier, that is, an imaging force, is reduced, and moreover, the toner tends to be separated from the carrier. As a result, scattering in a developing unit due to agitation, and fogging of a background portion tend to be caused.
During transfer, an electric field which is the reverse of a developing electric field is applied to facilitate the transfer of toner. However, in this case as well, when the charge of each toner particle is too small, transfer becomes difficult, resulting in unevenness or voids in a transfer image. Furthermore, regarding the adherence of the toner and the photoreceptor, or in other words, the force resulting from both the imaging force between the toner and photoreceptor, and the intermolecular force between the toner and photoreceptor, although imaging force decreases as the particle size is decreased, intermolecular force increases; accordingly, the ratio of non-electrostatic adherence increases. As a result, transfer by an transfer electric field becomes more difficult.
In order to obtain high quality images over a long period of time, it is very important to improve the charging properties of toner for which decreasing the particle size thereof has presented significant technical difficulties, and to achieve both improved development property and transferability, and furthermore inhibit the occurrence of in-machine contamination and fog over a long period of time.